CD80 is a member of the Ig superfamily and serves as the ligand for two T cell molecules，CD28 and CTLA4. Interactions between CD28 and CD80 on activated B cells result in enhanced T cell activation. CD80 is rapidly induced on the surface of in vitro activated B cells，Epstein Barr Virus (EBV) transformed B cell lines，Burkitts lymphoma cell lines，freshly isolated follicular B lymphoma cells，T cells，and monocytes. It is also expressed at high levels in dendritic cells. It reacts weakly with a small proportion of non activated normal B cells and with HTLV1 infected T cells. CD80 does not react with peripheral monocytes，resting and activated normal T cells，T cell lines and T cell clones，nor with myelomonocytic cell lines.